As known, a scanner can be used to perform a scanning operation after the scanner has been turned on for a warm-up time period.
The warm-up time period is attributed to a cold cathode fluorescent lamp (CCFL) used in the common scanner.
Referring to FIG. 1, a plot of the luminance variation of a cold cathode fluorescent lamp is illustrated. As shown in FIG. 1, in the initial stage from start of the scanner, the luminance of the cold cathode fluorescent lamp is slowly increased. Later, after the scanner has been turned on for a certain time period, e.g. T1 seconds, the luminance of the lamp approaches stable.
As known, the luminance value of the image data obtained when the scanner is operated in the unstable state is very different from that obtained when the scanner is operated in the stable state. For example, if the luminance value of the image data obtained in the stable state is assumed as 100, the luminance value obtained in the unstable state when the warm-up cycle of the lamp tube is not completed may be lower than 100, e.g. 70. Accordingly, for most scanners, a sufficient warm-up time period is necessary in order to achieve accurate image data.
Such a method of operating the scanner is not satisfactory because the scanning operation begins to be performed after the warm-up time period is due. Therefore, it is important to develop a method for performing the scanning operation without waiting for the warm-up time period.
Some scanning methods with no warm-up time period were disclosed. These scanning methods, however, employ specially designed circuits to control the luminance value of the image data by adjusting the reference voltage levels of the analog/digital conversion circuits built in the scanners.
Since the specially designed circuit is required for adjusting the reference voltage levels of the analog/digital conversion circuit built in the scanner so as to compensate the image data, the cost and the circuitry complexity of the scanner are increased.